In general, holographic recording for recording information in a recording medium utilizing holography is performed by overlapping light carrying image information and reference light in a recording medium and writing resultant interference fringes in the recording medium. When the recorded information is reproduced, the recording medium is illuminated with reference light to cause diffraction attributable to the interference fringes which reproduces the image information.
Recently, volume holography and, more particularly, digital volume holography has been developed and is attracting attention in practical fields for high density optical recording. Volume holography is a method for writing interference fringes on a three-dimensional basis by actively using a recording medium even in the direction of the thickness thereof, which is characterized in that diffracting efficiency is improved by an increased thickness and in that an increased storage capacity can be achieved utilizing multiplex recording. Digital volume holography is a computer-oriented method for holographic recording in which image information to be recorded is limited to binary digital patterns in spite of the fact that the same recording media and recording method as the volume holography are used. According to the digital volume holography, for example, analog image information such as a picture is once digitized to develop two-dimensional digital pattern information which is in turn recorded as image information. When reproduced, the digital pattern information is read and decoded to restore and display the original image information. Since this makes it possible to perform differential detection and error correction on encoded binary data, the original information can be reproduced with extremely high fidelity even with a somewhat poor SN ratio (signal-to-noise ratio) during reproduction.
FIG. 75 is a perspective view of a schematic configuration of a prior-art recording/reproducing system for digital volume holography. The recording/reproducing system has: a spatial light modulator 101 for generating information light 102 based on two-dimensional digital pattern information; a lens 103 for collecting the information light 102 from the spatial light modulator 101 to illuminate a hologram recording medium 100 with the same; reference light illumination means (not shown) for illuminating the hologram recording medium 100 with reference light 104 in a direction orthogonal to the information light 102; a CCD (charge-coupled device) array 107 for detecting reproduced two-dimensional digital pattern information; and a lens 106 for collecting reproduction light 105 emerging from the hologram recording medium 100 to illuminate the CCD array 107 with the same. Crystals of LiNbO3 or the like are used for the hologram recording medium 100.
In the recording/reproducing system shown in FIG. 75, recording is performed by digitizing information of an original image or the like to be recorded and by arranging the resultant signals having a value of 1 or 0 on a two-dimensional basis to generate two-dimensional digital pattern information. One piece of two-dimensional digital pattern information is referred to as “page data”. Let us assume here that page data #1 through #n are recorded in the same hologram recording medium 100 on a multiplex basis. In this case, the spatial light modulator 101 first chooses to transmit or block light for each pixel based on the page data #1 to generate spatially modulated information light 102 with which the hologram recording medium 100 is illuminated through the lens 103. Simultaneously, the hologram recording medium 100 is illuminated with reference light 104 in a direction θ1 substantially orthogonal to the information light 102 to record interference fringes resulting from overlap between the information light 102 and the reference light 104 inside the hologram recording medium 100. In order to improve diffracting efficiency, the reference light 104 is transformed by a cylindrical lens or the like into flat beams to record the interference fringes in the hologram recording medium 100 even in the direction of the thickness thereof. To record the next page data #2, the reference light 104 is projected at an angle θ2 different from θ1 and is overlapped with the information light 102 to perform multiplex recording of information in the same hologram recording medium 100. Similarly, to record the other page data #3 through #n, the reference light 104 is projected at respective different angles θ3 through θn to record information on a multiplex basis. Such a hologram having information recorded therein on a multiplex basis is referred to as “stack”. In the example shown in FIG. 75, the hologram recording medium 100 has a plurality of stacks (stack 1, stack 2, . . . , stack m, . . . ).
Arbitrary page data can be reproduced from a stack by illuminating the stack with reference light 104 at the same incident angle as that for the recording of the page data. As a result, the reference light 104 is selectively diffracted by interference fringes associated with the page data to generate reproduction light 105. The reproduction light 105 impinges upon the CCD array 107 through the lens 106, and the CCD array 107 detects a two-dimensional pattern of the reproduction light. The detected two-dimensional pattern of the reproduction light is decoded conversely to the process performed during recording so that information such as an original image is reproduced.
While the configuration shown in FIG. 75 allows multiplex recording of information in the same hologram recording medium 100, in order to record information with a high density, the positioning of the information light 102 and reference light 104 in the hologram recording medium 100 is important. In the configuration shown in FIG. 75, however, since the hologram recording medium 100 itself carries no information for positioning, there is only a mechanical way to position the information light 102 and reference light 104 on the hologram recording medium 100, which makes it difficult to perform the positioning with high accuracy. This has resulted in problems in that removability (the ease of performing recording and reproduction of a hologram recording medium on a recording/reproducing apparatus after moving it from another recording/reproducing apparatus with the same results as on the previous apparatus) is poor; random access is difficult; and high density recording is difficult. The configuration shown in FIG. 75 has another problem in that it involves a large optical system for recording or reproduction because the optical axes of the information light 102, reference light 104 and reproduction light 105 are located in different spatial positions.
Various methods for multiplex recording have been proposed in an attempt to increase the recording capacity of holographic recording by improving the recording density. One of such methods is angle multiplexing as shown in FIG. 75. However, such angle multiplexing has a problem particularly in that it involves a large and complex optical system for recording or reproduction because the angle of the reference light must be varied.
In addition to the above-described angle multiplex, proposed prior-art methods for multiplex recording for holographic recording include: phase-encoding multiplexing as disclosed, for example, in an article of J. F. Heanue et al., “Recall of linear combinations of stored data pages based on phase-code multiplexing in volume holography”, Optics Letters, Vol. 19, No. 14, pp. 1079–1081, 1994 and an article of J. F. Heanue et al., “Encrypted holographic data storage based on orthogonal-phase-encoding multiplexing”, Applied Optics, Vol. 34, No. 26, pp. 6012–6015, 1995; and hole burning type wavelength multiplexing as disclosed, for example, in an article by Eiji YAGYU et al., “A study on novel recording and reproduction of wavelength multiplex type holograms utilizing PHB”, Technical Report of IEICE, EDI93-87, HC93-54, pp. 1–5, 1993.
In any of the methods for multiplex recording, optical systems for recording or reproduction proposed in prior art have a problem in that their size is increased by the fact that the optical axes of information light, reference light and reproduction light are located in spatially different positions and in that a dramatic improvement in the recording density is not achievable because the hologram recording media themselves have no information for positioning and it is therefore difficult to position light for recording or reproduction on the hologram recording media with high accuracy.